1. Field
The present invention relates to a compound semiconductor structure and its manufacture method, and more particularly to a GaN series compound semiconductor structure using a conductive SiC substrate and its manufacture method. Here,
GaN series compound semiconductor is represented by AlxIny Ga1-x-yN (0≦x≦1.0, 0≦y≦1.0).
2. Description of the Related Art
Compound semiconductor devices using GaN or GaN series compound semiconductor are under vigorous development. GaN has a wide band gap of 3.4 eV and can operate at high voltages. Various semiconductor devices can be manufactured by forming a hetero junction using GaN series compound semiconductor. Metal organic chemical vapor deposition (MOCVD) is mainly used as a crystal growth method.
GaN has a high breakdown voltage, and is expected to be applied to usage requiring high voltage and high speed operation such as a high electron mobility transistor (HEMT) used at a base station of mobile phones. A variety of GaN-HEMTs have been reported which use as an electron transport layer a GaN layer of GaN/AlGaN crystals grown on a substrate such as sapphire, SiC, GaN and Si.
JP-A-2002-352956 proposes a GaN series HEMT having a high on-breakdown voltage and improved I-V characteristics, which is manufactured for example by laminating on a c-plane sapphire substrate an i-type GaN electron transport layer, an i-type AlGaN spacer layer, an Si-doped n-type AlGaN electron supply layer and an Si-doped n-type GaN protective layer by MOCVD, depositing an SiN layer by CVD, forming openings through the SiN layer, and forming source/drain ohmic electrodes and a gate Schottky electrode.
JP-A-2004-221325 proposes a HEMT which is manufactured by laminating on an SiC substrate an i-type GaN electron transport layer, an i-type AlGaN spacer layer, an Si-doped n-type AlGaN electron supply layer and an Si-doped n-type GaN cap layer, forming source/drain ohmic electrodes, thereafter depositing a first SiN protective film by CVD, forming an opening through the first SiN protective film, and forming a gate Schottky electrode, and further forming a second SiN protective film. This structure can suppress current collapse, i.e., a change in an on-resistance during operation.
A breakdown voltage in current-off state in excess of 300 V is presently reported. Best output characteristics are obtained by a GaN-HEMT using an SiC substrate. A high thermal conductivity of SiC contributes to this performance. When a high speed operation GaN device is to be manufactured, a semi-insulating SiC substrate is used in order to restrict parasitic capacitance.